Resistivity logging is a well-known form of electromagnetic (“EM”) propagation logging. Resistivity logging is used for measuring and evaluating the characteristics of potential hydrocarbon bearing zones in subsurface formations. Porous formations having high resistivity (low conductivity) generally indicate the presence of hydrocarbons, while porous formations having low resistivity (high conductivity) are generally water saturated. Impermeable shale formations also typically have lower resistivities than hydrocarbon-bearing formations. The ability to differentiate hydrocarbon-bearing formations from water-bearing formations or shale formations is particularly important in drilling horizontal wells where the goal is to maintain the drill bit within a relatively thin hydrocarbon zone.
In conventional wireline logging techniques, a number of antennae or coils are mounted on a well logging tool. The logging tool is lowered into a borehole on the end of a cable, or wireline. An alternating current energizes one or more transmitter coils to emit EM energy into the formations. The emitted energy propagates through the formations surrounding the borehole. The EM energy is measured by one or more receiver coils on the logging tool. The cable, which is attached to a mobile processing center at the surface, is the means by which measured data is sent up to the surface. With this type of wireline logging system, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is being pulled uphole.
U.S. Pat. No. 3,778,701, Method of Ultra Long Spaced Electric Logging of a Well Bore to Detect Horizontally Disposed Geologically Anomalous Bodies in the Vicinity of Massive Vertically Disposed Geologically Anomalous Bodies Lateral to and Not Intercepted by the Well Bore, describes a wireline logging technique whereby the presence of electrically resistive formations can be detected when they are not penetrated by the bore hole. An extended length electrode array injects current into the formation and measures the electric potential distribution along the electrode array. The resulting electric potential distribution can be interpreted to detect the nearby presence (or absence) of a zone of high resistivity zone, for example a salt dome. The method does not indicate the direction to the high resistivity zone.
An alternative to wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance. An especially important consideration is whether the drill bit is penetrating the correct geological formation.
Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling (“MWD”) techniques. Similar techniques focusing more on measurement of formation characteristics than on movement of the drilling assembly are known as logging-while-drilling (“LWD”). However, the terms MWD and LWD are often used interchangeably, and the use of either term in the present disclosure should be understood to include both the collection of formation and borehole information, as well as data on movement of the drilling assembly.
An emerging technique in the field of well logging is the use of tools incorporating tilted antennae, i.e., where the coils are tilted with respect to the tool axis. These apparatus are configured as such in an effort to alter the direction of the downhole measurement. U.S. Pat. No. 5,508,616, Apparatus and method for determining parameters of formations surrounding a borehole in a preselected direction, describes a logging tool incorporating tilted transmitter and receiver coils. U.S. Pat. No. 6,297,639 B1, Method and apparatus for directional well logging with a shield having sloped slots, describes an LWD logging technique especially suitable for drilling horizontal wells. U.S. Pat. No. '639 is assigned to the assignee of the present disclosure. The apparatus described in '639 contains antennas for transmitting and receiving EM energy. The receivers determine the relative attenuation and/or the phase shift of the EM energy propagating in the formation. When the apparatus is approximately parallel to a boundary between two formations having differing resistivities, the distance and direction to the boundary can be determined. This technology has proven to be extremely valuable since it allows the driller to maintain the horizontal well in the pay zone. Also see SPE paper 97045-MS, Deep Directional Electromagnetic Measurements for Optimal Well Placement, Q. Li et al, presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, 9-12 Oct. 2005, Dallas, Tex.
U.S. Pat. No. 4,323,848 A, Plural sensor magnetometer arrangement for extended lateral range electrical conductivity logging, describes a technique whereby a magnetometer is disposed in a wellbore to detect the distribution and distortion of an electric current flowing in an adjoining strata of interest. The current flow is excited by a distant alternating source located at the surface or at other boreholes, and perturbations in the current flow and accompanying magnetic field caused by the electrical characteristics of the adjoining strata are registered by the magnetometer.
U.S. Patent Application No. 20070126426 A1, Method and Apparatus for Locating Well Casings from an Adjacent Wellbore, is assigned to the assignee of the present disclosure. It describes a technique for locating a casing from a second wellbore.
The apparatus includes an electric current driver having an insulated gap and a magnetometer positioned within a non-magnetic housing that is disposed within a non-magnetic tubular. The electric current enters the formation and a portion of it concentrates on a nearby conductive casing, which results in a secondary magnetic field being generated. The magnetometer is disposed in such a manner that it selectively senses the secondary magnetic field. The distance and direction from the apparatus to the casing is determined from the magnetometer's measurements. This invention has an important application to drilling SAGD (Steam Assisted Gravity Drainage) well pairs in heavy oil formations. The pair of horizontal SAGD wells must be drilled parallel to each other, at a predetermined spacing, typically 5 meters, and with one well placed exactly above the other.
It is desirable to obtain an apparatus and method that can detect the presence of a formation of different resistivity which is not penetrated by the borehole, and to determine the direction and distance to the said formation. Furthermore, such apparatus is not limited to the use of tilted coils, which results in more cost effective and reliable system.